PermutationMap

[dannys4]

I'd like a map that allows me to shuffle coordinates. This obviously can be done with an Affine map, but that's a frustrating amount of BLAS overhead for something that's far more "embarrassingly parallel". If we have permutation map $S(x)=Px$ for some matrix $P$, of course, we have
$$S^{-1}(x)=P^\top x$$
$$\log|\nabla S(x)| = \log|P| = 0$$
$$\nabla_x\log|\nabla S(x)| = 0$$

Of course, similar to AffineMap, we don't include the permutation in the varied parameters and thus there are no coefficients.

This would be a great straightforward contribution for someone getting started with the codebase.